Is it valid for someone who caught one rakaʿah of Maghrib prayer with the congregation to stand up after the taslīm and pray two continuous rakaʿāt, believing that the middle tashahhud is a Sunnah and does not invalidate the prayer?

If the latecomer (masbuq) believes that what he caught up with is the end of his prayer, then there is no harm in what he did. This is a view held by some scholars. However, if he believes that what he caught up with is the beginning of his prayer, and he deliberately stood up from the tashahhud, then there is a difference of opinion regarding the invalidity of his prayer. It is invalid according to the Hanbalis if the omission was deliberate and with knowledge of the ruling. It is valid according to the Malikis, according to the well-known opinion.